ArF excimer laser based microlithography has been extensively used by the semiconductor industry to mass-produce patterned silicon wafers. The industry constantly demands more performance from excimer laser sources. As a result, greater demands are constantly places on excimer laser optical components, for example, the highly reflective mirrors that are used in 193 nm wavelength excimer lasers that at high repetition rates.
As semiconductor processing has progressed from the 65 nm to the 45 nm node and beyond, the application of optical coatings in the spectral regime of deep ultraviolet (DUV) has been extended and is now used for laser optics (the optical components used in connection with excimer laser based systems; for example, the highly reflective mirrors that are used in 193 nm wavelength excimer lasers that at high repetition rates) and precision optics (for example, the reticule inspection objective). Regarding laser optics, the optical components are exposed to high laser fluence. As a result, laser durability of laser optics is one of the main challenges to the industry. For precision optics, on the other hand, an objective or projection system comprises many lenses with various surface curvatures, and low-loss antireflection (AR) coatings are of extreme importance for such application. Generally, at least one high refractive index and one low refractive index fluoride material are required for making 193 nm optical coatings.
Among the very limited number of materials that can be used for such mirrors, GdF3 and LaF3 are considered as high refractive index materials and MgF2 and AlF3 are the low refractive index materials that are used for wavelengths below 200 nm. [See D. Ristau et al., “Ultraviolet optical and microstructural properties of MgF2 and LaF3 coating deposited by ion-beam sputtering and boat and electron-beam evaporation”, Applied Optics 41, 3196-3204 (2002); R. Thielsch et al., “Development of mechanical stress in fluoride multi-layers for UV-applications”, Proc. SPIE 5250, 127-136 (2004); C. C. Lee et al., “Characterization of AlF3 thin films at 193 nm by thermal evaporation”, Applied Optics 44, 7333-7338 (2005); R. Thielsch et al, “Optical, structural and mechanical properties of gadolinium tri-fluoride thin films grown on amorphous substrates”, Proc. SPIE 5963, 5963001-12 (2005); and Jue Wang et al, “Nano-structure of GdF3 thin film evaluated by variable angle spectroscopic ellipsometry”, Proc. SPIE 6321, p 6321071-10 (2006)].
At the present time there is renewed research interest in wide band-gap fluoride thin films for ArF laser optics applications. The application of energetic deposition processes, such as plasma ion-assisted deposition (PIAD), ion assisted deposition (IAD) and ion beam sputtering (IBS), are restricted for fluoride materials because of the nature of fluoride materials. As a result, the industry has accepted thermal resistance evaporation (TRE) for fluoride film deposition without introducing fluorine depletion. However, when thermal resistance evaporation is used as the film deposition method, the resulting fluoride film packing density is low (that is, it is porous) and the film structure is inhomogeneous. Neither of these is desirable because a porous structure can harbor environmental contamination and increases scatter losses. Various approaches have been applied to improve fluoride film structure, including substrate temperature and deposition rate. Recently, the effect of substrate crystal orientation has also been taken into account, but no significant improvements have been reported. [see Y. Taki and K. Muramatsu, “Hetero-epitaxial growth and optical properties of LaF3 on CaF2”, Thin Solid Films 420-421, 30-37 (2002), and US patent 200302276670 A1 to Y. Taki et al., titled “Optical Element Equipped with Lanthanum Fluoride Film”].
Another problem arises from the fact that many periods of high index and low index layers (one period equals one high and one low refractive index layer) are required in order to get high reflectivity at 193 nm, for example, in highly reflective mirrors. However, the surface/interface roughness and inhomogeneity increase as number of layers and the overall thickness increases. The control of the multilayer fluoride film structure is critical for achieving high reflectivity at 193 nm. In addition to their use in microlithography, fluoride coated mirrors are also required for those areas where ArF excimer laser have other, non-lithographic application, for example minimal invasive brain-, vascular- and eye surgery; ultra-precision machining and measurement; large-scale integrated electronic devices; and components for communication. In view of the problems extant with the present fluoride coated elements (for example, mirrors and other laser system optical elements) that are used in below 200 nm, for example, 193 nm, lithography, it is desirable to have fluoride coated elements that overcome these problems. In addition to mirrors, the invention can also be applied to beamsplitters, prisms, lenses, output couplers and similar elements used in <200 nm laser systems.